(a) Field of the Invention
The present invention relates to a hydrophobic composition and a method of preparing a hydrophobic plate, and specifically, to a hydrophobic composition having good hydrophobic property which can give a good hydrophobic property to a plate though a small amount of hydrophobic resin is used and a method of preparing a hydrophobic plate using the composition.
(b) Description of the Related Arts
In general, a sealed nickel-hydrogen cell, that is an alkali storage battery uses metal oxide as an cathode and hydrogenated alloy as a anode. This cell is charged or discharged by storing the hydrogenated alloy with hydrogen ions produced by decomposing water in the electrolyte during the charging process, and releasing hydrogen ions into the electrolyte during the discharging process.
The nickel-hydrogen cell uses hydrogenated alloy as a negative active material, Ni(OH).sub.2 as a positive active material, and aqueous alkali solution as an electrolyte. The nickel-hydrogen cell can be represented by a general formula of a Ni--MH (nickel-metalhydride) cell. Electro-chemical reactions of the nickel-hydrogen cell are represented by the chemical equations: EQU Cathode:Ni(OH).sub.2 +OH.sup.- .revreaction.NiOOH+H.sub.2 O(1) EQU Anode:M+H.sub.2 O+e.sup.- .revreaction.MH+OH.sup.- (2)
wherein, M is a hydrogenated alloy that can absorb and release hydrogen ions
A charge-discharge cycle is performed according to the above chemical reaction. When the cathode is overcharged, oxygen gas is electro-chemically generated in the cathode, and when the anode is overcharged, hydrogen gas is electro-chemically generated in the anode. When the charge-discharge cycle is continuously performed under the above condition, the gases generated cause the anode to oxidize and the internal pressure of a cell to rapidly increase thereby decreasing the lifetime and the capacity of the cell. Accordingly, hydrogen and oxygen gas generated should be removed in the cell. In this regard, the anode acts to remove hydrogen and oxygen gas. The chemical reaction for removal of gas is represented by the following equation: EQU Chemical reaction for removal of gas: 2MH+1/2O.sub.2 2M+H.sub.2 O(3)
The anode in a nickel-based cell absorbs and then decomposes oxygen and hydrogen gas, thereby decreasing the internal pressure of the cell. Accordingly, it makes to facilitate oxygen decomposition in the anode. Methods of facilitating oxygen decomposition are as follows: adding a catalyst for reducing oxygen; increasing a surface area to be reacted by etching an alloy; or treating a anode plate with hydrophobic material. Among the above methods, the method which can increase the interface of the anode, at which the cleavage or absorption of gas has occurred, by treating the anode with a hydrophobic material in order to give a hydrophobic property to the anode, is widely used.
When the anode is treated with a hydrophobic material according to the above method, it has an advantage that the internal pressure of a cell is decreased, however, a disadvantage is caused that the internal resistance of the cell is increased because the hydrophobic material is a nonconductor. Furthermore, when the plate is too densely coated and the thickness of the coated plate is too thick, it is difficult to react a gas that reached the anode with the anode. In addition, when the surface is not uniformly coated over the anode, cell reaction does not uniformly occur in the anode, thereby deteriorating the performance of the cell. Accordingly, in the method wherein a anode is treated with a hydrophobic material to decrease the internal pressure of the cell, it is preferable to use a material having a good hydrophobic property as a hydrophobic agent. Further, it is important to uniformly and thinly coat fluoro-based resin having a hydrophobic property on the surface of the anode.
At present, in order to give a hydrophobic property to a plate, a method is employed wherein fluoro-based resin powder as a hydrophobic material is dispersed in an organic solvent such as alcohol, or aqueous solution, and then the dispersion obtained is sprayed on a plate.
According to this method, when fluoro-based resin powder is sprayed on the plate, a three-phase (vapor-liquid-solid) interface is formed on the surface of the plate so that gas readily attaches on the plate, and it thus provides the advantage of absorbing or decomposing gas on the plate to decrease the internal pressure.
However, the disadvantages of the spray method is that it is difficult to uniformly coat the dispersion of the fluoro-based resin on the plate, and an excess amount of the resin can be partly coated on the plate. Therefore, conductivity of the plate is lowered and it is difficult to form a pore. Moreover, in case of using alcohol as a solvent, alcohol vaporizes to produce an alcoholic vapor which is harmful to humans and is extremely flammable in mass production. Furthermore, the spray method has problems when the hole of the spray nozzle clogs up with the resin of the spray solution, thereby decreasing the operation efficiency. In addition, when hydrophobic resin is not uniformly coated, cell reaction can not be uniformly performed over a anode plate by continuing a charge-discharge cycle, so that the performance of cell is deteriorated.
In order to solve problems as described above, recently a method is employed wherein a plate is dipped into a fluoro-based resin dispersion to give a hydrophobic property to the surface of the plate.
However, the method of dipping the plate into a fluoro-based resin dispersion has problems of involving a complicated process which comprises the steps of dipping the plate into a hydrophobic resin, drying and calendering using a roller.
The conventional methods of giving a hydrophobic property to a plate as described above, have problems as the following: the effect obtained by giving a hydrophobic property to the plate, is not sufficient because the methods give a hydrophobic property to only the surface of the anode. Furthermore, electrolyte does not smoothly move from the cathode to the anode by continuing a change-discharge cycle, and electrolyte in the anode is exhausted, thereby decreasing the capacity of the cell. Moreover, unevenness on the surface of the anode and stains are occurred by the surface tension of the solution in drying.